As a reference, I like the sound of Momentum over ear.  I don't like Grado Sr80i which is too bright for me. 
 
Comfort:  Excellent.  I worried the headband design will not be comfortable but after a week of using X2, I think this is probably the best system for such a large and relatively heavy headphone.  Unlike the usual headband design, the weight of the headphone is evenly distributed on the head. I don't feel the headphone being heavy at all. There is virtually no or very little clamping force and yet the headphone is very secure.  Momentum is much more comfortable because of its light weight and low clamping force.  X2 has slightly less clamping force than HD558 so it is slightly more comfortable.
 
Sound:  Compare with HD558, X2 is better in all frequency range.  X2 extend to lower bass and higher treble.  Momentum have a better bass impact but X2 can provide a deep rumble at lower end which is quite satisfying.  I find X2 a bit on the bright side.  It is nowhere near as bright as Sr80i.  I agree with review by PowderLegend that the sound of cymbals, snares, drum brushes are a bit artificial.  This is the only area that I find X2 quite disappointing.  
 
Overall, I like X2.  It has open sound as you expect from an open headphone.  Unlike other open headphones (HD558 and Sr80i), it has very good bass -- not bass head bass.  The only downside is the slightly artificial treble.